Most likely culprit are the indent springs, however in order to eliminate a couple of other possibilities the first thing you need to do is get a full SMG re-adaption, not the half baked one that the cheap hobbyist systems do, as they often are only capable of doing the clutch. This will eliminate the GPT and possibly even cure the problem if it's only GPT out of cal. If the adaption fails and tells you to replace the gearbox then it's almost guaranteed to be indent springs, which are cheap, but a pain to replace. Ask on Cutters and you'll likely find someone nearby who's done it.

Be aware that when you do the adaption, if it fails, things may get much worse, to the point of being nearly undriveable, giving all the symptoms of a badly slipping clutch. Panic not, replace the indent springs and do the adaption again- most important, and it'll be sublime.